Hades became Artemis's favourite uncle when she was around seventeen.
One of her hunting dogs, her dogs usually as free-spirited and strong-willed as Artemis is herself, was running amok in Olympus during preparation for a party and causing destruction, and the other gods were tripping over themselves trying to grab it without disturbing Artie (it was her birthday party, first one since she'd claimed her title as Goddess of the Hunt, and she was getting ready.) They were not succeeding. Until Hades arrived.
He watched the scene with amusement for a moment before deciding to help; whistling so sharply that Artie peeked out of her room to see what the source of the sound was, then snapping his fingers by his side and ordering "Here" in a tone of such finality that the dog trotted right over to his side and sat down. He proceeded to call the dog a good girl and scratched her behind the ears, and gave the dog the first genuine smile Artemis had ever seen from him.
When she expressed surprise, he told her that he had "around three" dogs at home (whether Cerberus counts as one dog or three is a matter of constant debate in his house.) Hades loves dogs, he's absolutely a dog person, and he knows how to handle them. Then he handed her a gift, said "Happy Birthday," and left. He wasn't staying for the party- he's never been a social god.
And from that moment on, he's been her favourite.